A conventional method of plastic surface pretreatment prior to metallization consist of etching the surface with solution containing permanganate or other Mn ions which are the etchants-oxydizers. Permanganate as oxidizing agent is commonly proposed to be used in the acidic media, because only in this case the surfaces of acrylo-nitryl-butadien-styrene copolymere (ABS), ABS mixed with polycarbonate (PC/ABS), polyetherimides (PEI) and polyetheretherketones (PEEK) are able to be modified during the etching with certain functional groups allowing to obtain good metal adhesion with said plastics (U.S. Pat. No. 3,647,699A, 1970; US20040096584A1, 2003; WO2005094394A2, 2005; US20090176022A1, 2006; US20110140035A1, 2007; U.S. Pat. No. 9,023,228B2, 2008; EP2825689A1, 2013).
The main shortcoming of the above mentioned methods is that the stability of permanganate ions in the acidic media is enough limited, especially if such media contains strong (sulphuric, nitric, chlorate) inorganic acids. Even phosphoric acid with concentration more than 45% (vol) compared to the rest of the water is not recommended (US20050199587A1, 2004; WO2005094394A2, 2005). However, if the etcher contains just phosphoric acid with concentration not exceeding 50 vol %, the oxidizing properties of the permanganate ions at a room temperature are too weak for many plastics including mentioned above. Therefor the such etcher can work properly just at temperature, at least up to 50-60° C. However, the stability of permanganate ions greatly reduces at elevated temperature (accelerates their decomposition reaction, releasing the oxygen) therefore the concentration of permanganate ions decreases rapidly to unacceptable limits and, despite the etchers higher temperature, it becomes unable to modify chemically the plastic's surface properly (US20050199587A1, 2004; U.S. Pat. No. 9,023,228B2, 2008).
It is also a known method of using a strongly acidic media to stabilize permanganate ions (WO2014124773A2, 2014). This method is based on the fact, that enough concentrated sulphuric acid forms strong complex compound with permanganate ions. However, highly concentrated sulphuric acid chemically intensively reacts with many plastics, in particular PEEK, destroying plastic's surface structure. Thus, it is impossible to get a good coating adhesion to plastics using such conditions. In addition, the permanganate ions tend to form insoluble Mn2O7 in concentrated sulphuric acid, which remains on the etcher's surface. Mn2O7 is extremely strong oxidizer, which in contact with the organic compounds can be self-ignite or even cause an explosions.
The method for plastics surface pretreatment comprising etching of the surface with permanganate ions in the solution of phosphoric acid which contains also nitric acid (US20090176022A1, 2006) is the closest to the proposed invention. Nitric acid enhances the efficiency of the etcher and enables to obtain good adhesion of metallic coating to plastics etching at room temperature. However, even under such conditions, nitric acid, being a strong inorganic acid, considerably accelerates the self-decomposition of the permanganate ions and the problem of the etcher's stability remains unsolved.